There are four vortex generators on each side of the rear fuselage above the horizontal stabiliser. These are to reduce elevator and elevator tab vibration during flight to increase their hinge bearing service life. It is also probable that they energise the airflow at the stagnation point at the tailcone thereby reducing drag and giving a slight performance advantage.

Only aircraft after line number 2277 (May 1992 onwards) have vortex generators fitted at production but there are many later 737-200's with and 737-300's without (see photos). The CDL says that if they are not fitted or any are missing "occasional vertical motions may be felt which appear to be light turbulence and is not to be confused with mach buffet."

Quoting HAWK21M (Reply 5):Anyone having a history behind these,or were they installed on the 1st B731s produced.

It's really unlikely that they were designed in (purist aerodynamicists hate things like that). They look an awful lot like a fix found in flight test. Vortex generators are very easy to install, have almost no weight and drag penalty, and can fix many kinds of annoying problems.

Quoting OPNLguy (Reply 6):One could conclude that they've been there from day-1....

Quoting Tdscanuck (Reply 7):It's really unlikely that they were designed in (purist aerodynamicists hate things like that). They look an awful lot like a fix found in flight test.

I guess some Aerodynamic problem during tess warranted them.
If only Manufacturers printed a book on how & what caused them to adjust certain features on a type Aircraft in its history.something which the AMM does not tell.
regds
MEL

The fuselage narrows at this point at a quite substantial angle. That increases the local airspeed quite a lot. (many planes have a more pointed aft fuselage than the 737, and therefore don't accelerate the air as much).

When the plane is going fast the local airflow can go supersonic, or just about sonic, or transonic.

The vortex generators creates a vortex in the boundary layer in such a way that at least the boundary layer remains subsonic. That way slight vibrations in the control surfaces are eliminated.

Without those vortex generators, then at least when making a minor sideslip (in turbulent air) some chock-waves could slam a little on the rudder and elevator.

The same vortex generators are seen on many planes on the upper surface of the wing in front of the ailerons where the serve the same purpose. More seldom they are seen on the lower surface on the horizontal tail in front of the elevator hinge.

(Sorry for the improper linking - the proxy here is acting up and won't allow me to use the buttons above. The usual {/img} isn't working either.

Also, apropos of nothing, I was interested to find when doing a search for other searches of D-ABEI, pictures of a 733 as well. I had no idea that they recycled registrations that way, (I'm not a spotter), though it's obvious they would have to, now that I think about it.

I have never worked on a 737 that didn't have them. I can tell you this, those things will tear your skin off. Funny that this topic is here today. I was on a rudder change last night and those things bit me 20 times if not more. They are incredibly pointy (engineer word) on the edges. Disconnecting the rudder pcu and the lower 3 hinges put your knees, hands, head in direct contact with them and they will draw blood. I'm no engineer but just looking at them, they look like they are designed for postive airflow across the rudder and to reduce turbulence on the same, I could be completely wrong here though.

Hawk21M: You ain't kidding. I got 13 stitches in my noggin a couple of years ago from the gear door. Was repacking a gear and stood up in the wrong place. I have a very bad habit of locating sharp objects with my head. You would think I would learn, but these danged airplanes just keep getting in the way. That door will get you pretty regularly when changing the 1 or 4 tire. Later

You'll see vortex generators in other places on the 737, the largest being the "horn" on the inboard side of the engines. Here you can see them in action (whispy vortex over the wing) along with the 4 on the rear fuselage and one on the vertical stab itself:

Here are 8 short ones on the upper wing that keep control authority for the outbord flaps:

There are also 3 small VGs on the leading edge that I think help the ailerons:

Quoting Prebennorholm (Reply 9):The fuselage narrows at this point at a quite substantial angle. That increases the local airspeed quite a lot. (many planes have a more pointed aft fuselage than the 737, and therefore don't accelerate the air as much).

When the plane is going fast the local airflow can go supersonic, or just about sonic, or transonic.

The vortex generators creates a vortex in the boundary layer in such a way that at least the boundary layer remains subsonic. That way slight vibrations in the control surfaces are eliminated.

Quoting Tdscanuck (Reply 12):I must be missing something here...diverging duct in subsonic airflow should result in a local airspeed decrease, not increase.

I'm gonna have to agree with Tdscanuck on this one. What I suspect happens is that the tapering fuselage does indeed cause the subsonic airflow around the tail cone to slow down. From Bernoulli's law, we know that if the velocity of flowing air decreases, the static pressure increases correspondingly.

The boundary layer of this airflow is thus moving against an adverse pressure gradient, an adverse pressure gradient being one where static pressure increases as we move along a surface. The boundary layer, or layer of air adjacent to the skin which already has reduced velocity and hence energy, can eventually be slowed to a stop and even forced to reverse direction due to this adverse pressure gradient.

At this point, the boundary layer will separate from either side of the aft fuselage surface. Between the separated streamlines on either side of the fuselage, will be a large low pressure region (wake) with a highly unsteady vortex shedding characteristic. The low pressure region causes pressure drag, but of more concern is the vortex shedding and the turbulence in generates.

The vortex generators alleviate this problem by generating stream-wise vortices. These vortices force high velocity and high momentum air outside the boundary layer to mix with the sluggish air inside the boundary layer. This re-energises the boundary layer and allows it to travel much further against the adverse pressure gradient before it separates. If the boundary layer separates off a bluff body (aft fuselage) further along the surface, the size of the low pressure wake region and severity of the vortex shedding will be less.

Regards, JetMech

JetMech split the back of his pants. He can feel the wind in his hair.

Quoting ZuluLima (Reply 18):Here are 8 short ones on the upper wing that keep control authority for the outbord flaps:

I'm not sure that's what they're for...the flaps aren't really control surfaces and they're slotted, so they get most of their boundary layer boost from high-pressure air coming off the bottom of the wing. Someone stated in another thread that those VG's were there to reduce transonic drag effects, although I don't know the physics behind that.

Quoting ZuluLima (Reply 18):There are also 3 small VGs on the leading edge that I think help the ailerons:

In cruise, they look like they're below the stagnation point so they wouldn't have any significant effect on the ailerons. They may be to help the slats during low-speed/high-angle flight.

Quoting JetMech (Reply 19):What I suspect happens is that the tapering fuselage does indeed cause the subsonic airflow around the tail cone to slow down.

I think JetMech's explanation is dead on, and consistent with the aero guys I've talked to at Boeing. The 737 has a very strong fuselage taper at the back end (much more than any other Boeing or Airbus) that can lead to separation.